linux-kernel - [PATCH] cpufreq: pcc-cpufreq: Disable dynamic scaling on many-CPU systems

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Date:   Tue, 17 Jul 2018 14:02:29 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     linux-pm@...r.kernel.org
Cc:     Andreas Herrmann <aherrmann@...e.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] cpufreq: pcc-cpufreq: Disable dynamic scaling on many-CPU systems

From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

The firmware interface used by the pcc-cpufreq driver is
fundamentally not scalable and using it for dynamic CPU performance
scaling on systems with many CPUs leads to degraded performance.

For this reason, disable dynamic CPU performance scaling on systems
with pcc-cpufreq where the number of CPUs present at the driver init
time is greater than 4.  Also make the driver print corresponding
complaints to the kernel log.

Reported-by: Andreas Herrmann <aherrmann@...e.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---
 drivers/cpufreq/pcc-cpufreq.c |    8 ++++++++
 1 file changed, 8 insertions(+)

Index: linux-pm/drivers/cpufreq/pcc-cpufreq.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/pcc-cpufreq.c
+++ linux-pm/drivers/cpufreq/pcc-cpufreq.c
@@ -589,6 +589,14 @@ static int __init pcc_cpufreq_init(void)
 		return ret;
 	}
 
+	if (num_present_cpus() > 4) {
+		pcc_cpufreq_driver.flags |= CPUFREQ_NO_AUTO_DYNAMIC_SWITCHING;
+		pr_err("%s: Unsuitable system, dynamic performance scaling disabled\n",
+		       __func__);
+		pr_err("%s: Change BIOS settings and complain to the hardware vendor\n",
+		       __func__);
+	}
+
 	ret = cpufreq_register_driver(&pcc_cpufreq_driver);
 
 	return ret;